1. Field of the Invention
The present invention relates to a combustion state detecting device that detects a combustion state of an internal combustion engine by detection of a change in the quantity of ions which are produced at the time of burning in the internal combustion engine, and more particularly to a combustion state detecting device for an internal combustion engine which is capable of diversifying detection functions by producing a plurality of currents analogous to an ion current to be detected.
2. Description of the Related Art
In general, in an internal combustion engine driven by a plurality of cylinders, the fuel-air mixture consisting of air and fuel introduced into the combustion chambers of the respective cylinders is compressed by moving up pistons, electric sparks are generated by applying an ignition high voltage to ignition plugs located in the respective combustion chambers, and an explosion force developed at the time of burning the fuel-air mixture is converted into a piston push-down force, to thereby extract the piston push-down force as an rotating output of the internal combustion engine.
There has been known that since molecules within the combustion chambers are ionized when the fuel-air mixture has been burned within the combustion chambers, ions having electric charges flow between the ignition plugs as an ion current upon application of a bias voltage to ion current detection electrodes (as usual, ignition plug electrodes are used) located within the combustion chambers.
Also, there has been known that the combustion state of the internal combustion engine can be detected by detection of a state in which the ion current occurs because the ion current is sensitively varied according to the combustion state within the combustion chambers.
FIG. 6 is a structural diagram showing one example of a conventional combustion state detecting device for an internal combustion engine.
In the figure, one end of a primary winding 1a of an ignition coil 1 is connected to a power supply terminal VB whereas the other end thereof is connected to the ground through a power transistor 2 having an emitter thereof grounded, which serves as a switching element for interrupting the supply of a primary current I1.
A secondary winding 1b of the ignition coil 1 constitutes a transformer in cooperation with the primary winding 1a, and a high-voltage side of the secondary winding 1b is connected to one end of an ignition plug 3 corresponding to each cylinder (not shown) to output a high voltage of negative polarity at the time of controlling ignition.
Each ignition plug 3 made up of counter electrodes is applied with an ignition high voltage to discharge and fire the fuel-air mixture within each of the cylinders.
It should be noted that the ignition coil 1 and the ignition plug 3 are disposed in parallel for each of the cylinders, however, in this example, only one pair of ignition coil 1 and ignition plug 3 are representatively shown.
A low-voltage side of the secondary winding 1b is connected to a bias circuit 6 through a resistor 4 and a diode 5 which are connected in parallel and constitute current limiting means.
The resistor 4 suppresses a discharge current that flows into the ignition plug 3 through the secondary winding 1b from the bias circuit 6 and suppresses a voltage developed at the high-voltage side of the secondary winding 1b at the time of starting the supply of the current to the primary winding 1a.
The diode 5 is provided so that a direction in which the secondary current (ignition current) flows at the time of applying the ignition high voltage becomes forward, and is arranged so as to suppress a potential difference between both ends of the resistor 4 at the time of controlling ignition.
The bias circuit 6 applies a bias voltage of a polarity reverse to the ignition polarity, that is, the positive polarity to the ignition plug 3 through the resistor 4 and the secondary winding 1b to substantially detect an ion current corresponding to the quantity of ions generated at the time of burning.
The bias circuit 6 is connected to a current-voltage converter circuit 7, and the current-voltage converter circuit 7 converts the ion current allowed to flow by the bias voltage into a voltage and applies the voltage thus converted to a voltage signal distributor circuit 8 as an ion current detection signal.
The voltage signal distributor circuit 8 distributes the ion current detection signal (ion signal) which has been converted into a voltage to a knock detection signal generator circuit 9 that extracts a knock signal from the ion signal and a combustion/misfire signal generator circuit 10 that produces a signal used for judging combustion/misfire according to the ion signal, respectively.
Then, output signals from the knock detection signal generator circuit 9 and the combustion/misfire signal generator circuit 10 are supplied to an ECU (electronic control unit) 11. The ECU 11 judges the combustion state of the internal combustion engine on the basis of the output signal from the combustion/misfire signal generator circuit 10, and conducts adaptive control appropriately so as not to cause inconvenience when detecting the deterioration of the combustion state.
Also, the ECU 11 arithmetically operates an ignition timing, etc., on the basis of drive conditions obtained from a variety of sensors (not shown) such as the knock detection signal generator circuit 9 or a crank angle sensor 12 to output not only an ignition signal V1 to the power transistor 2 but also a fuel injection signal to an injector (not shown) for each of the cylinders and drive signals to a variety of actuators (a throttle valve, an ISC valve, etc.)
FIG. 7 is a circuit structural diagram showing an example of a specific circuit structure of the bias circuit, the current-voltage converter circuit and the voltage signal distributor circuit shown in FIG. 6.
In the figure, the bias circuit 6 includes a capacitor 6a connected to a low-voltage side of the secondary winding 1b through the resistor 4 and the diode 5 which are connected in parallel, a diode 6b disposed between the capacitor 6a and the ground, and a Zener diode 6c for limiting bias voltage which is connected in parallel with the capacitor 6a.
A series circuit consisting of the capacitor 6a and the diode 6b and the Zener diode 6c connected in parallel with the capacitor 6a are disposed between the low-voltage side of the secondary winding 1b and the ground through the diode 5 to constitute a charging path for charging the capacitor 6a with the bias voltage at the time of generating the ignition current.
The capacitor 6a is charged with the secondary current flowing therein through the ignition plug 3 which is discharged at a high voltage outputted from the secondary winding 1b when the power transistor 2 is off (when the supply of the current to the primary winding 1a is interrupted). The charge voltage is limited to a predetermined bias voltage (for example, about several hundreds V) by the Zener diode 6c and substantially functions as bias means for ion current detection, that is, a power supply.
A resistor 7a which is connected in parallel with the diode 6b and serves as the current-voltage converter circuit 7 converts the ion current allowed to flow by the bias voltage into a voltage, and supplies the voltage thus converted to the voltage distributor circuit 8 as the ion current detection signal.
The voltage signal distributor circuit 8 includes a plurality of buffers 8a and 8b, and the output side of the buffer 8a is connected to the knock detection signal generator circuit 9 while the output side of the buffer Bb is connected to the combustion/misfire signal generator circuit 10.
Subsequently, the operation of the conventional combustion state detecting device for an internal combustion engine shown in FIGS. 6 and 7 will be described with reference to FIGS. 8A to 8F.
In general, the ECU 11 arithmetically operates the ignition timing, etc., in accordance with the drive conditions, and supplies an ignition signal V1 (FIG. 8A) to the base of the power transistor 2 at a targeted control timing to control the on/off operation of the power transistor 2.
As a result, the power transistor 2 interrupts the supply of the primary current I1 (FIG. 8B) flowing in the primary winding 1a of the ignition coil 1 to boost the primary voltage, and also develops the ignition high voltage, that is, the secondary voltage V2 (FIG. 8C) of, for example, several tens kV at the high-voltage side of the secondary winding 1b.
The secondary voltage is applied to the ignition plug 3 in each of the cylinders and allowed to generate a discharge spark within the combustion chamber of the ignition control cylinder to burn the fuel-air mixture. At this time, if the combustion state is normal, a required quantity of ions are generated in the periphery of the ignition plug 3 and within the combustion chamber.
Then, as described above, when the power transistor 2 is turned on in response to the ignition signal V1, the supply of the current to the primary winding 1a starts, to thereby develop the voltage with the positive polarity at the high-voltage side of the secondary winding 1b.
At this time, since the discharge current from the capacitor 6a to the low-voltage side of the secondary winding 1b is limited by the resistor 4, the voltage developed at the secondary winding 1b is divided to the high-voltage side and the low-voltage side without being superimposed on the bias voltage.
At the time of starting the supply of a current to the primary winding 1a, even if the voltage of the positive polarity is developed at the high-voltage side of the secondary winding 1b, since the discharge current from the capacitor 6a to the low-voltage side of the secondary winding 1b is limited by the resistor 4 as described above, the voltage of the positive polarity developed at the high-voltage side of the secondary winding 1b is suppressed so that there is no case in which the ignition plug 3 discharges.
Sequentially, at the time of interrupting the primary current, if the ignition high voltage is developed at the high-voltage side of the secondary winding 1b to make the ignition plug 3 discharge, the secondary current I2 (FIG. 8D) then flows in a path of the ignition plug 3, the secondary coil 1b, the diode 5, the capacitor 6a, the diode 6b and the ground in the stated order to charge the capacitor 6a with a given voltage V3 (FIG. 8E).
When the charge voltage of the capacitor 6a reaches a given voltage value of the Zener diode 6c, the secondary current flows into the Zener diode 6c without flowing into the capacitor 6a, to thereby maintain the given bias voltage.
Upon the completion of the discharge by the ignition plug 3, the charge voltage of the capacitor 6a is applied to the ignition plug 3 through a path of the resistor 4 and the secondary coil 1b in the stated order so that the ion current flows in a path of the capacitor 6a, the resistor 4, the secondary coil 1b, the ignition plug 3 (ions in the ignition plug gap), the ground, the resistor 7a and the capacitor 6a in the stated order. The ion current is converted into a voltage by the resistor 7a to produce an ion signal SI (FIG. 8F).
The ion signal is distributed by the buffers 8a and 8b of the voltage signal distributor circuit 8, and the ion signal from the buffer 8a is supplied to the knock detection signal generator circuit 9 where a knock signal is produced. Also, the ion signal from the buffer 8b is supplied to the combustion/misfire signal generator circuit 10 where a combustion/misfire signal is produced.
Then, the output signals from the knock detection signal generator circuit 9 and the combustion/misfire signal generator circuit 10 are supplied to the ECU 11, and the ECU 11 produces and outputs a variety of control signals such as the above-described ignition signal and drive signals on the basis of the detection signal from those output signals and the detection signals from a variety of sensors (not shown) such as the crank angle sensor 12.
In the conventional combustion state detecting device for an internal combustion engine structured as described above, because the resistor is disposed in the path into which the ion current flows to conduct voltage conversion when the ion current is converted into a voltage, one dynamic range of the ion signal is determined by that resistor. However, the quantity of ion current is greatly different depending on the drive state of the internal combustion engine, and the peak value of the ion current is within a range of from several to several hundreds .mu.A. Accordingly, there arise such problems that it is very difficult to conduct signal processing for detection of knocking, detection of combustion/misfire and detection of other combustion states, and also that a signal processing circuit at a post-stage becomes very complicated, etc.